The invention concerns a hydraulic valve system with a supply connection arrangement, having a high-pressure connection and a low-pressure connection, with a working connection arrangement, having two working connections, with a control valve, which is arranged between the supply connection arrangement and the working connection arrangement, and with a non-return valve for each working connection, said valve being arranged between the control valve and the working connection in question, a tappet mechanically opening the valve element of said non-return valve.
A hydraulic valve system of this kind is known from DE 40 28 887 A1. Here, the tappet for opening the non-return valve in question is activated via a slant on the slide of the control valve. Through the opening of the non-return valve, a returning of the motor to a non-deflected or non-extended state will also be possible, when only a small load, for example the dead weight, acts upon the motor, this load being so small that it cannot provide the pressure required across some auxiliary valves to open the non-return valve.
A hydraulic valve system with non-return valves is particularly intended for mounting on tractors or other kinds of mobile equipment. The non-return valves are meant to ensure a substantially drop-free behaviour, that is, they should retain the motor connected to the working connection arrangement in a certain position, when this has been set so by the control valve. The non-return valves are opened by means of a pilot pressure. This appears from, for example, DE 199 31 142 A1 or 199 19 015 A1. However, it is necessary that the motor can produce a sufficient pilot pressure.
With double-acting motors (or other consumers) this is usually no problem, as, when being moved in one direction by the pressure at one working connection, the motor automatically produces a correspondingly large pressure at the other working connection, which is then used to open the non-return valves. With single-acting motors, this is not the case. As fluid is only available on one side of the motor, it can only be activated in one direction. In order to be able to revert to a starting position, it has to be acted upon by a load in the opposite direction.
Basically, it is not known for sure in advance, which consumer is to be connected to the valve arrangement. Therefore, both with double acting motors and with single-acting motors, the valve arrangement must be able to open the non-return valves. When, for example, a tractor linkage drawbar is controlled by the valve arrangement, however, no weight resting on the drawbar, then there is hardly no load for lowering the drawbar. The pressure in the tank line would have to be close to 0 bar, which is usually very hard to realise.
Therefore, in DE 40 28 887 A1 a forced opening of the non-return valves is provided. When the slide of the control valve is moved in one direction, it produces the hydraulic connections required to operate the motor in one direction. At the same time, the non-return valve is opened via the tappet and the slide, to enable a discharge of the fluid from the opposite pressure chamber of the motor.
However, this embodiment has some disadvantages. Firstly, a relatively large friction occurs between the slide of the control valve and the tappet of the non-return valve, which is, for example, a disadvantage, when the slide is to be electrically activated. Further, with this embodiment a partly substantial wear can be seen, which involves the risk that eventually the valve arrangement will become unable to work.